1. Field of the Invention
The present invention relates to a control system for controlling a continuously variable transmission on the basis of parameters indicating an engine output torque.
2. Related Art
The continuously variable transmission to be mounted on an automobile is exemplified by a continuously variable transmission (as will be called the "CVT") which is constructed by combining a metallic belt and a pair of (primary/secondary) pulleys having a variable groove width, for example.
In this CVT, the speed or RPM (Revolutions Per Minute) of an engine, as inputted from the primary pulley, is changed with the change in the winding diameter of the belt made to run between the two pulleys, and is transmitted from the secondary pulley to the differential and the drive wheels. Moreover, a gear ratio is controlled by adjusting the flow of oil fed to the primary pulley.
On the other hand, the torque, transmitted between the primary pulley and the secondary pulley, is determined by the force for pushing the belt between those pulleys. In the CVT, therefore, a line pressure control is made to push the belt sliplessly with the secondary pulley by using an oil pressure, for example. As this line pressure grows higher, the force to push the belt becomes the higher. If a pushing force higher than necessary is applied to the belt, however, the smooth movement of the belt is deteriorated. Therefore, the line pressure control is performed under a target value which is determined on the basis of the input torque inputted to the CVT (i.e., the torque outputted from the engine) and the gear ratio.
This line pressure control is made in the prior art on the basis of a torque information dictated by an average effective pressure (which is calculated by dividing the torque by the displacement of the engine and which indicates a parameter value of the engine output torque) calculated on the basis of a demanded value of an engine output demanded by the driver, by using a value estimated from the average effective pressure as the engine output torque.
Here, the torque information, obtained on the basis of the accelerator opening, is not the information of the torque which is generated from the engine by burning a fuel, but the preceding torque information which was intended to generate the power. Thus, the torque information has a quick output timing and an excellent responsiveness for a shift in which the torque abruptly changes. However, the torque information has a poor precision because it is estimated from the value demanded by the driver.
In a steady state with little torque change, therefore, because of a poor precision of the torque information, the frequency of generating an excessive belt pushing force is so high that the fuel economy (i.e., miles/gallon) of the automobile is adversely affected.